This invention pertains to electromagnetic discharge lamps and, more particularly, is concerned with fluorescent lamps.
Perhaps the most familiar electromagnetic discharge light source is the common fluorescent lamp. Usually the lamp has a cylindrical envelope filled with a low pressure inert gas and a small dose of metallic mercury. Voltage, when applied to electrodes within the envelope, accelerates electrons which ionize the inert gas, initiating a discharge. Heat and electrons from the discharge vaporize and excite the mercury which emits ultraviolet and visible radiation, with a strong ultraviolet line at 253.7 nm. A phosphor layer inside the envelope converts the ultraviolet to visible light.
Many modifications have been proposed to improve the conventional fluorescent lamp which is less compact then an incandescent lamp of equal luminescence. Departing from a straight tube configuration, envelopes have been formed into toroids, spheriods, re-entrant cavities, and many other configurations. Beam shaping electrodes have been demonstrated, as have electrodeless discharges. Most of these modifications, however, call for mercury in the discharge medium.
Effort has also been made to improve the filling. For examples, U.S. Pat. No. 4,427,921 issued Jan. 24, 1984 to Proud et al for "Electrodeless Ultraviolet Light Source" disclosed fillings including I, HgI.sub.2, and CdI.sub.2, and U.S. Pat. No. 4,427,922 issued Jan. 24, 1984 to Proud et al for "Electrodeless Light Source" describes fillings including HgI.sub.2, HgBr.sub.2, and HgCl.sub.2.
In the related art of high pressure mercury vapor lamps it has been known for a number of years to improve the visible output of such lamps by adding metal halides to a filling of mercury and inert gas. U.S. Pat. No. 3,586,898 "Aluminum Chloride Discharge Lamp" issued to Speros and Simper divulges a filling of aluminum trichloride, mercury, and inert gas with the optional addition of aluminum tri-iodide. The lamp's envelope is alumina or alumina coated quartz to avoid reaction between AlCl.sub.3 and SiO.sub.2. Krefft et al divulges in U.S. Pat. No. 2,135,718 a high pressure mercury lamp with an inner tubular container and an outer envelope.
Mercury and cadmium are known to accumulate in biological systems and are hazards to human health. While the dosage of these metals expected from individual lamps is likely to be below the threshold of harm, it would be desirable to avoid their use if an alternative efficient fill material were available.
Copending U.S. patent application Ser. No. 402,175, filed July 26, 1982 for Mercury-Free Discharge Lamp divulges an ultraviolet lamp which contains a filling of aluminum trichloride and one or more inert gas. Copending U.S. patent application Ser. No. 401,528, filed July 26, 1982 for Compact Mercury-Free Fluorescent Lamp discloses a fluorescent lamp containing a mercury-free fill which includes aluminum trichloride.
Accordingly, it is an object of this invention to provide an efficient fluorescent lamp specifically arranged to use a filling of aluminum tribromide and one or more inert gases. Another object is to provide a fluorescent lamp more compact than conventional fluorescent lamps.